This invention pertains to a combustor designed to provide a non-toxic method of completely combusting organic or inorganic materials by using a dual cyclone to recirculate particulate matter.
The disposal of waste vegetation (i.e., trees, brush, yard waste, etc.) and other organic materials is a major concern of municipal, commercial, and private sectors. Various techniques are currently used to dispose of such waste. The most common technique has been burying waste in landfill sites. However, landfill sites are becoming scarce and cost-prohibitive due to rapidly expanding urban areas. See U.S. Pat. No. 5,415,113.
One alternative to landfills is incineration. An incinerator is a device that uses high temperature combustion to produce relatively complete oxidation of the waste material. The efficiency of combustion can be increased by maximizing mixing. Mixing has important effects on heat and mass transfer and on chemical reactions. See S. Zabrodsky, Hydrodynamics and Heat Transfer in Fluidized Beds, (M.I.T. Press, Cambridge, 1966). Incineration (combustion) is one of the most widely used treatments of hazardous waste, offering the following advantages: (1) volume reduction, (2) detoxification, (3) environmental impact mitigation, (4) regulatory compliance, and (5) energy recovery. See W. Niessen, Combustion and Incineration Processes, (Marcel Dekker, Inc., New York, 1978). Additionally, incineration of waste vegetation produces an ash residue high in natural nutrients that are beneficial to plant growth. When the ash is mixed with compost and varying amounts of soil, a range of products including high-grade potting soil and top soil can be produced.
As compared to other waste treatment methods, incineration achieves the highest overall destruction and control for the broadest range of waste streams. Therefore, incineration is gradually replacing the disposal of wastes in landfills. See C. Lee et al., xe2x80x9cIncinerability Ranking Systems for RCRA Hazardous Constituents,xe2x80x9d Hazardous Waste and Hazardous Materials, vol. 7, no. 4, pp. 385-415 (1990). The environmental hazards of burning trash in barrels or other types of open burning are not present with proper incineration. Unlike backyard open fires, which burn in the range of 200-300xc2x0 C., resulting in incomplete combustion, municipal waste incinerators burn at temperatures over 1000xc2x0 C. and add enough oxygen to achieve essentially complete combustion. Many dangerous compounds can be completely eliminated at these high temperatures, while eliminating smoke and odor. See Lee et al., 1990.
The primary objective of waste combustion is to destroy the organic and pathogenic constituents in the waste streams, leaving behind an inert residue with minimum carbon content. To be a successful waste management option, combustion must accomplish this goal in a cost-effective and fuel-efficient manner, without creating significant risks from emissions. See R. Seeker, xe2x80x9cWaste Combustion,xe2x80x9d Twenty-third Symposium on Combustion/The Combustion Institute, pp. 867-885 (1990).
The simplest definition of combustible waste is material that has primarily an organic content and that can be oxidized by combustion. Three features of the waste generally determine the combustion characteristics and type of equipment that is suitable. These include the average physical and chemical characteristics of the waste, any special constituents in the waste streams, and the variability of the waste properties.
Several parameters have been found to increase combustion efficiency, including high temperature and excitation of particles by acoustic vibrations. See Seeker, 1990; I. Glassman, xe2x80x9cCombustion,xe2x80x9d Academic Press, 2nd ed., pp. 386-409 (1987); and J. Willis et al., xe2x80x9cAcoustic Alteration in a Dump Combustor Arising From Halon Addition,xe2x80x9d Combustion and Science and Technology, vol. 94, pp. 469-481 (1993). Another important factor in efficient combustion is recirculation of incompletely burned particles. One way to increase recirculation is to use a cyclone separator. Cyclone separation occurs when air and waste enter tangentially at the top of the tube and descend with a generally circular motion described by an outer vortex. During the downward descent, the heavier material travels along the periphery of the tube and is thus separated from the lighter xe2x80x9cclean air.xe2x80x9d See S. Henderson et al., Agricultural Process Engineering, (John Wiley and Sons, Inc., New York, 1955).
Combustion is a complicated process. A complete analytical description of a combustion system requires consideration of the following factors, among others: (1) chemical reaction kinetics and thermodynamics under nonisothermal, heterogeneous, and nonsteady conditions; (2) fluid mechanics in nonisothermal, heterogeneous, reacting mixtures, with heat release that can involve laminar, transition, turbulent, plug, recirculating, and swirling flows within geometrically complex enclosures; and (3) heat transfer by conduction, convection, and radiation between gases, liquids, and solids with high heat release rates and (with boiler systems) high withdrawal rates.
One important physical parameter in waste incinerator design and operation is the character of the waste feed. Waste materials can include a wide spectrum of physical forms, e.g., pumpable liquids, sludge, slurries, tarry semi-solids, contaminated soils, solid refuse (paper, plastic), and bulky solids. The physical characteristics largely dictate the method used to introduce the waste into the device and the combustion chamber configuration employed. See Seeker, 1990.
Another key parameter that dictates the design and operation of combustion systems for a particular form of waste is the presence of any special constituents that can influence system operation or performance, e.g., lead to pollution formation, retard the flame, form fine salt particles, or cause corrosion.
When combusting organic materials such as wood, several factors must be considered, including the xe2x80x9cglobalxe2x80x9d molecular formula, the low heat value in the dry-ash-free state, and the heat of formation. The xe2x80x9cglobalxe2x80x9d molecular formula of wood is about C6H9O4. In the dry-ash-free state, the heat value ranges between 4200 and 4500 kcal/kg, depending on the wood species. A standard heat of formation for wood is xe2x88x92188 kcal/mol at 25xc2x0 C. Wood that has been naturally dried in ambient air stabilizes its moisture content at about 20 percent. As wood is heated, it first gives off primarily water vapor. When temperatures reach about 275xc2x0 C. or above, fuel gases are produced that spontaneously burn in air between 450 and 650xc2x0 C., a process called pyrolysis. After pyrolysis, the residual carbon remaining (probably due to an insufficient amount of oxygen) represents about 15 to 30 weight percent of the initial wood. The rate of the thermal degradation, as well as the nature and quantities of the various products, depend on the temperature. The overall kinetics depend on the size of the wood particles. See A. Beenackers, Advanced Gasification, (Kluwer Academic, Massachusetts, 1986).
Stoichiometric combustion of typical wood is described by the following reaction:
C6H9O4+6.25O2xe2x86x926CO2+4.5H2O
Temperature, one of the most important parameters in combustion processes, is difficult to measure and control. Temperature variability inside an incinerator is caused by many factors, including wall radiation, flow velocity, and oxidation reactions on wall surfaces.
The presence of sound waves in a dump combustion chamber has been shown to increase the rate at which particles decompose. The acoustic vibrations cause a higher rate of mixing of particles and oxygen, producing a reduced combustion time. See J. Willis et al., xe2x80x9cDestruction of Liquid and Gaseous Waste Surrogates in an Acoustically Excited Dump Combustor,xe2x80x9d Combustion and Flame, vol. 99, pp. 280-287 (1994).
It has also been determined that resonant acoustic conditions in dump combustors can materially increase the rate of heat release, resulting in high volumetric heat release rates, i.e., high power in a compact device. Under resonant conditions, chemistry, fluid mechanics, and acoustics are tightly coupled. Thus, an incinerator that takes full advantage of resonant operation must be designed to handle changes in heat release rates or characteristic chemical reaction times. See Willis et al., 1994.
In prior studies, three different acoustic modes were identified with combustion operation, including frequencies in the 600-700 Hz range, the 400-600 Hz range, and the 30-50 Hz range. See Willis et al., 1994. The level of waste destruction can be strongly influenced by the acoustic mode. For example, operation in the lowest frequency mode results in levels of destruction two orders of magnitude lower than that observed in high frequency modes. See Willis et al., 1994.
U.S. Pat. No. 5,944,512 describes an incineration device for use in heating applications in which the process stream is re-circulated. The device comprises a tangential blower inlet, a single exhaust outlet at the top, and a vertical, conical-shaped heating chamber where the apex of the cone is near the flame at the bottom.
U.S. Pat. No. 5,415,113 describes a portable incineration device for disposing waste vegetation, comprising a box-shaped combustion chamber and a manifold assembly adapted to direct a curtain of high velocity air across the top opening of the combustion chamber. The high velocity air is directed down into the combustion chamber and then exits at the top.
U.S. Pat. No. 5,361,710 describes a compact waste incinerator that improves combustion efficiency through the active production, placement, and stabilization of large scale vortices within the combustion chamber, coupled with the controlled and synchronized injection of fuel and waste relative to the large scale vortices.
U.S. Pat. No. 5,193,490 describes a circulating fluidized bed boiler that uses a horizontal, cylindrical-shaped chamber with multiple inlets to induce cyclonic mixing and combustion.
U.S. Pat. No. 5,123,361 describes an annular vortex combustor that burns highly viscous fuel, for example, ultra-fine coal, pulverized coal, or coal water fuel. A vertically oriented vortex is created when fuel and atomizing air are injected tangentially near the bottom of combustion chamber. The vortex motion is maintained by injecting secondary air through nozzles vertically distributed along the length of the combustion chamber.
U.S. Pat. No. 5,111,757 describes a cylindrical containment vessel that is defined by stable fluid recirculation, maintained by the superposition of at least two vortices produced by a combination of nozzles and blowers.
U.S. Pat. No. 4,565,137 describes a solid bio-mass fuel burner with a specialized delivery system for injecting solid fuel into a combustor involving multiple air injectors that help create a cyclonic vortex. The vortex is maintained by tangentially injecting air into the combustion chamber through a plurality of passages horizontally distributed along the longitudinal axis of the chamber.
U.S. Pat. No. 4,144,019 describes a double vortex, horizontal burner that comprises both a cylindrical outer chamber and a cylindrical inner wall that terminates at a cone-shaped end. Walls separate the two vortices.
U.S. Pat. No. 3,855,951 describes a cyclonic incinerator that combines a vertically oriented cyclone separator, a combustion chamber with an inclined or conical kiln device, and a recirculation flow line for greater combustion and particulate removal efficiency.
U.S. Pat. No. 3,777,678 describes a fuel burner having a horizontally oriented circular chamber into which air and fuel are tangentially injected, creating a cyclone movement that is maintained by injecting air through a plurality of openings along the length of the chamber.
U.S. Pat. No. 2,707,444 describes a cyclonic furnace that creates a cyclonic vortex by injecting entrained fuel and air through one or more tangential inlets located near the top of the chamber. Small combusted particulates exit an axial outlet at the top of the chamber.
We have discovered an induced-vortex combustor that provides a method to combust organic or inorganic materials. The device comprises a vertical combustion chamber with a conical top and an air exhaust that exits through the bottom. The initial fuel rests on a conical mesh just above the burner. Air flows into the chamber below the screen to suspend the materials as it burns. The device also allows loading of material during combustion through two loading bins, whereas other combustors must be shut down to add additional material. Optionally, acoustical devices can be added to aid mixing.
Unlike prior incinerators or solid fuel combustors that rely on secondary air to maintain a vortex, the novel device creates a vortex system comprising two vortices that increase mixing of material through the continuous injection of air tangentially near the base of the combustion chamber and increase the combustion of particles through recirculation. The initial injection of air creates a horizontal, outer ascending vortex that is converted to a descending, inner vortex by the conical top, resulting in a double vortex system.